1. Field of the Invention
The present invention relates to a mass spectrometer and a method of mass spectrometry.
2. Discussion of the Prior Art
Inductively Coupled Plasma mass spectrometry (ICP-MS) is commonly used for trace element analysis. A number of different forms of ICP-MS systems are known.
Quadrupole ICP-MS is a high throughput system that offers a limited mass to charge ratio resolution capability typically (m/z)/Δ(m/z) equal to 400. High resolution ICP-MS uses a combination of electrostatic and magnetic sectors in order to significantly increase the mass to charge resolution up to about (m/z)/Δ(m/z) equal to 10000.
Time of Flight ICP-MS is a variant of the low resolution ICP-MS system wherein a Time of Flight mass analyser is used instead of a quadrupole mass analyser in order to achieve high speed acquisition. Reaction/collision cell ICP-MS is another variant of the quadrupole ICP-MS system wherein a reaction/collision cell is provided prior to the quadrupole mass analyser in order to achieve selective reduction of spectroscopic interferences. Ion trap ICP-MS is a yet further variant of the low resolution ICP-MS system wherein an ion trap mass analyser is used instead of a quadrupole mass analyser in order to achieve a greater control over the reduction of spectroscopic interferences through gas molecule reactions.
A significant problem with the known mass spectrometers is that the detection limits of a number of elements analysed by quadrupole and Time of Flight ICP-MS systems are limited by spectroscopic interferences.
An example of particular interest is Phosphorus which is a mono-isotopic element that can be detected as P+ ions having a nominal mass to charge ratio of 31. At the same nominal mass interfering molecular ions such as NOH+, 13C18O30, C18OH+, 13COH2+ and COH3+ are also often observed. The intensity of the interfering molecular ions may be particularly high especially when solvents rich in nitrogen, oxygen or carbon are used. This inevitably leads to poor detection limits for Phosphorus.
Although it is possible using high resolution ICP-MS to resolve the above mentioned interferences, this is generally at the expense of sensitivity. Furthermore, high resolution ICP-MS systems are relatively large and expensive.
It is known to attempt to address the problem of interferences using reaction/collision cell ICP-MS systems. Instead of separating analyte and interference ions based on their mass to charge ratio differences, gases are added to a reaction/collision cell to provide a means for separating analyte ions from interfering ions based upon their energy differences. The ions formed in the plasma have different energies from molecular ions formed elsewhere or reaction by-products formed in the reaction/collision cell. Therefore, by using the reaction/collision cell in an energy-filtering mode it is possible to separate plasma ions from other ions. Whilst this approach does offer some improvement in detection limits compared with standard quadrupole ICP-MS systems, the energy difference approach often results in a substantial loss of sensitivity.
Ion trap ICP-MS systems can be used to refine the control of the reaction processes required to separate interfering ions from analyte ions. However, due to the storage limitations of ion traps these systems have a relatively low sensitivity particularly when the population of the analyte ions is very small compared with the interfering ions For example, Phosphorus and Sulphur are particularly difficult to detect when organic solvents are used since interferences are normally particularly intense.